1. Field of the Invention
The present invention relates to an automatic gain control circuit for a video processing system and, more particularly, to an automatic gain control circuit which automatically controls gain by using a sound gain controller so as to stably output a video intermediate frequency signal.
The instant application is based on Korean Patent Application No. 93-31322, which is incorporated herein by reference for all purposes.
2. Discussion of Related Art
Typically, an automatic gain control circuit is used, for example, in the television arts to automatically control the gain of an amplifier in a receiving apparatus, preventing changes in output level of the amplifier even when an input level thereto is changed. The receiving apparatus may be an audio or a video system, as well as a communication apparatus.
FIG. 1 is a circuit diagram of a conventional automatic gain control (AGC) circuit. When a video intermediate frequency signal is input through an input terminal P1, a gain control amplifier 1 amplifies the input video intermediate frequency signal by a video gain control voltage and then outputs an amplified signal. A gain control voltage detecting circuit 2, which receives the amplified video intermediate frequency signal from the gain control amplifier 1, detects a gain control voltage from a video carrier signal and outputs the detected gain control voltage to a lockout stopping circuit 3. A video detecting circuit 4, which receives the amplified video intermediate frequency signal from the gain control amplifier 1, detects a video signal and outputs the detected video signal. The lockout stopping circuit 3, which receives the detected video signal from the video detecting circuit 4, compares the detected video signal with a predetermined reference level value and then detects whether the comparing result corresponds to a normal state or an abnormal state. If a normal state is detected, the gain control voltage detected from the gain control voltage detecting circuit 2 is output to the gain control amplifier 1. If an abnormal state is detected, a predetermined reference voltage is applied to the gain control amplifier 1. As a result, lockout occurring when the gain is controlled can be prevented.
The conventional AGC circuit, as described above, has the disadvantage wherein an accurate gain control voltage cannot be detected because the gain control voltage is derived from a video intermediate frequency signal. The AGC circuit is ineffective in that a sample and a hold circuit must be added thereto although the gain control voltage is detected using a synchronizing signal.